This invention relates to method and apparatus for measuring brightness of illumination and, more particularly, to a method and apparatus for expressing as a number the relative brightness of artificial illumination as it is perceived by the average observer.
Early in this century, experiments with so-called flicker photometers were interpreted to mean that the normal human visual system responds to light of different wavelengths in accordance with what is now known as the standard photopic curve. The need for some sort of detector to place values on brightness measurements was very great and the photopic curve was adopted throughout the world as if it were indeed an accurate measurement of human vision under ordinary conditions. This curve as adopted forms the basis of the lumen and the footcandle, and hence is now the international standard for artificial illumination. A meter or detector which is provided with this response is said to measure luminance, expressed in footcandles, or the metric unit lux.
Unfortunately, a detector such as a footcandle meter, which is provided with the response of the photopic curve, usually disagrees with a human observer in the comparison of two different illumination sources when the spectral power distributions of the two sources differ. As stated by W. D. Wright in "The Measurement of Colour", fourth edition (1969) Van Nostrand Reinhold, page 66, "The embarrassing situation can arise that a fluorescent lamp A of one colour may look brighter than a lamp B of slightly different colour, although the lumen output of B as measured photometrically is greater than that of A". In other words, artificial lighting sources would be ranked in one way by their measured luminance, and in a different way by their brightness as perceived by an average observer.
Another problem with the footcandle meter is that it is unfailingly additive. Thus, fifty footcandles of one illuminant, superimposed on fifty foot candles of any other illuminant, will be measured by the meter as a total of one hundred footcandles. Again, the experts in the field do not agree with this law of additivity, which necessarily applies to footcandle measurements, but does not apply to perceived brightness. In explanation, white light, upon removal of the red component, becomes bluish-green and brighter to the average observer than the original white light. Thus, by removing red light from a white mixture, a brighter light is produced. The footcandle meter would disagree with the observer in this experiment also, since the removal of the red-footcandle component from the white light can only reduce the footcandle content of that light, even though its brightness increases as perceived by the average observer.
A third problem encountered with the footcandle meter can be illustrated by illuminating identical furnished rooms with two commercial lamps which have the same source color, but different spectral power distributions. If the two rooms are adjusted to have the identical footcandle luminance, one room will normally appear at least somewhat brighter than the other to the average observer.
Still another problem with the footcandle meter is that when apparent brightnesses of different colors of illumination are involved, very substantial differences between the relative values of luminance and perceived brightness may exist. As an example, yellow-appearing illumination which is adjusted to provide the same footcandle luminance as violet-appearing illumination will be much dimmer in perceived brightness. According to established experiments, this difference in perceived brightness may be as great as 975%, even though the footcandle meter reads both sources as having equal footcandle levels, see P. L. MacAdam comments in Illuminating Engineering, February 1958, page 70.